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Source Case Investigation

In some cases a contact investigation will be done with patients, especially young children with TB infection or noninfectious disease. This is called a source case investigation. The source is a person with infectious TB disease who transmits M. tuberculosis to another person or persons. The patient is identified through a contact or source case investigation and may or may not be the index patient.

The Purpose of Source Case Investigations

Source case investigations are recommended for young children age 2 and under with latent TB infection and age 4 and under with active TB disease.3 This is because TB in young children indicates recent transmission, so the ability to locate an infectious source case is more likely. Not all source case investigations result in potentially infected contacts being identified. The ultimate goal of a source case investigation is to determine who transmitted M. tuberculosis to a child, index patient, or persons in a cluster of skin test conversions and whether this person

Is still infectious;

Was reported to the health department as a TB case or suspect; or

Has infected others

Even though not routinely recommended for adults, source case investigations for adults may also be conducted in some health departments, especially when there is

A group of documented recent tuberculin skin test conversions in a high-risk institution;or

A severely immunosuppressed person who does not have a known history of latent TB infection found to have TB disease.

Identifying Associates

In source case investigations, the names the interviewer elicits are called associates. For child source case investigations, emphasis is placed on adults or adolescents with whom the child spends or has spent great amounts of time. In such a scenario, there is no infectious period, since children rarely spread TB. Health department guidelines should be followed for establishing a time frame to guide the elicitation process. A time frame of greater than 6 months can make it difficult for individuals to recall exposure sites and people they have had contact with.

Since young children may not be the best source of information, the parent, guardian, or someone who knows the child’s lifestyle well should be interviewed. A source case interview should match the format of a standard TB interview, with the exception of symptom elicitation to verify an exposure time frame. Questions for source case investigations regarding children should be asked about associates in the following areas:

Household (those living in the household, frequent visitors, other homes where the index patient has lived)

School

Regular child care (both inside the home and outside the home)

Car pools

Play groups

Recreational activities

Extended family and friends

Places of recent travel

The interviewee should also be reminded of confidentiality and the contact referral process. Education must be provided, particularly on transmission and the difference between latent TB infection and TB disease.